The present invention concerns OLED display devices. More specifically, the present invention concerns display devices known as passive matrix organic light emitting diodes or PMOLEDs. In PMOLED display devices, lighting is controlled by means of anodes and cathodes which are disposed in strips perpendicular to each other and powered separately. Although less energy efficient than active matrix light emitting diodes, also known as AMOLEDs, PMOLED display devices are less complex to make than AMOLED display devices and consequently were the first to be mass marketed.
Compared to other types of display devices such as liquid crystal cells, OLED display devices have very interesting advantages such as a very rapid response time, improved colour rendering, better contrast or even less directive emissivity, thus offering a broader angle of vision. Compared to liquid crystal display devices, another decisive advantage of OLED display devices is that they do not require backlighting. However, one drawback of OLED display devices is their sensitivity to air and moisture.
Very succinctly summarised, a PMOLED display device includes a base substrate, for example made of glass, on which anodes are structured in parallel strips. These anodes are made of a transparent, electrically conductive material such as indium tin oxide also known as ITO. After the indium tin oxide layer has been deposited on the glass substrate and the anodes have been structured, there is deposited in succession an insulating layer, whose purpose is to separate the line electrodes and column electrodes from each other, and a separation layer whose role is to create a space between the anodes and the cathodes to be deposited. The actual active layers, of which there are three, are then deposited: a hole injection layer, a hole transport layer and an electron transport layer. Finally, a layer of conductive material such as aluminium is deposited, in which the cathodes are structured. The resulting structure is covered with a cover which is bonded onto the edge of the base substrate. A moisture trap may be arranged in the volume left vacant by the cover.
As mentioned above, OLED display devices have a certain number of very interesting advantages for use, for example, in portable electronic objects, such as wristwatches. However, in the particular field of horology, those skilled in the art have been confronted by a problem, which, to the Applicant's knowledge, has not yet been resolved. This problem is connected to the combined use, in a timepiece, of a digital OLED display device and an analogue display device which conventionally includes a set of hour and minute hands. Indeed, in that case, the OLED display device is on the watch dial and the hour and minute hands move above the OLED display device. The hour and minute hands are carried by a set of concentric arbours which must necessarily pass through the OLED display device. However, since the active layers and the cathode layer of an OLED display device are very sensitive to air and moisture, it is not possible to envisage piercing the OLED display device for the arbours carrying the hour and minute hands to pass therethrough, since this would place the moisture-sensitive layers of the OLED display device in direct contact with the atmosphere.